A crossover story could be really exciting. In this kind of fanfiction, we might see the Survey Corps from Attack on Titan having to deal with the ghouls from Tokyo Ghoul. Maybe Eren and his friends would be confused at first about these new, strange creatures that are part human, part something else. They could initially mistake the ghouls for Titans. And the ghouls, on the other hand, would be equally shocked by the Titans and the advanced technology the Attack on Titan characters use. It would be a clash of two very different but equally dark and violent worlds.
Well, in this crossover, the Survey Corps members like Armin would be fascinated by the intelligence of some ghouls. They might try to communicate with them to gain knowledge about their powers. Ghouls like Touka would be in a difficult situation. She would be in a world full of titans, which are much larger and more difficult to deal with than the CCG in her own world. She might have to learn new fighting techniques from the Survey Corps. And characters like Erwin would see the ghouls as a valuable asset, trying to recruit them to fight the titans.
The Survey Corps' ODM gear could be used in new and creative ways in a crossover. They could use it to quickly move around the Tokyo Ghoul cityscape, giving them an advantage against the ghouls. Meanwhile, the ghouls' kagune could be used to attack Titans in ways the Attack on Titan characters haven't seen before. For example, a ghoul could use its kagune to slice through a Titan's nape from a distance.
The characters would interact in very interesting ways. For instance, Levi from Attack on Titan might be initially suspicious of the ghouls. But once he sees their combat abilities, he could try to recruit them or at least work with them. Kaneki, on the other hand, might be in awe of the Survey Corps' determination. He could learn a lot from their teamwork and discipline. And the ghouls' concept of a 'family' within Anteiku could be something that the soldiers of the Survey Corps might find strange but also endearing.
One interesting plot twist could be the ghouls in the Attack on Titan world. Imagine if ghouls were hidden among the titans. They could feed on the bodies of dead titans or even on the soldiers without being detected at first. Another could be the Ackermans having some sort of connection to the ghoul powers. Maybe their enhanced strength is related to a dormant ghoul gene. And a third could be the Scouts finding a way to use ghouls' kagune to fight the titans more effectively, like creating weapons out of kagune material.
A cool plot twist could be a ghoul - Titan hybrid. Imagine a Titan with the regenerative abilities of a ghoul. This would pose a whole new level of threat to the characters from both universes. Also, the idea of the ghouls' coffee being a valuable resource in the Attack on Titan world could be an interesting twist. Since coffee is so important to ghouls, it could have some unforeseen effects on the Titans or the people in that world.
A 'Black Butler' and 'Tokyo Ghoul' crossover fanfiction could have Sebastian from 'Black Butler' serving CCG investigators in 'Tokyo Ghoul'. He could use his demonic powers to hunt down ghouls in a very stylish and efficient way, much like how he serves Ciel in his own series. His loyalty to the investigators would be a fascinating aspect to explore, as well as how the investigators react to his otherworldly abilities.
It would be epic! Imagine Eren Yeager with the powers of the Hulk. He could transform into a titan - Hulk hybrid, with the strength of both. The Avengers would be both confused and intrigued by the existence of Titans. Maybe Captain America would try to form an alliance with the Survey Corps to fight a common enemy that threatens both their worlds.
The storyline could be really interesting. Maybe the ghouls could be a new kind of dark creature in the Harry Potter world. The characters from Tokyo Ghoul, like Ken Kaneki, might attend Hogwarts. He could struggle with his ghoul nature while trying to fit in with the wizards and witches. The magic in the Harry Potter universe could also have an impact on the ghouls' abilities, perhaps suppressing or enhancing them.
One possible plot could be a character from Tokyo Ghoul ending up in the world of Vampire Knight. Maybe Kaneki, with his unique ghoul powers, could interact with the vampires and hunters in Vampire Knight. He might initially be seen as a threat, but as the story progresses, they could find common ground in their struggle for survival in a world that fears them.
Well, one possible plot could be that the Titans somehow cross into the Avengers' universe. Maybe Thanos' snap disrupted the fabric of reality and opened a portal to the Attack on Titan world. The Avengers, seeing these giant, menacing Titans, would initially be shocked. Iron Man could start analyzing the Titans' weaknesses while Captain America rallies the team to fight. Meanwhile, Eren Yeager and the Survey Corps might find themselves in New York City and team up with the Avengers. They'd share their combat skills against the Titans, and Eren could show off his Titan - shifting abilities, which would be a huge asset to the Avengers.